The present invention relates to an improvement in the emulsion polymerization of a dienic monomer or a monomeric mixture containing a dienic monomer in an aqueous medium for the purpose of reducing or preventing the deposition of polymer scale on the inner walls of a polymerization reactor and other surfaces in contact with the monomer or monomeric mixture.
Emulsion polymerization is the most widely practiced technique for the preparation of a variety of synthetic rubber latices including styrene-butadiene rubber, acrylonitrile-butadiene rubber and chloroprene rubber as well as certain synthetic resins including acrylonitrile-butadiene-styrene and methylmethacrylatebutadiene-styrene, in which a dienic monomer is involved. However, in the prior art emulsion polymerization in an aqueous medium, one important problem is that large amounts of polymer scale will deposit on the inner walls of the polymerization reactor and other surfaces in contact with the monomer or monomers during the course of the polymerization. As a result, the cooling efficiency of the reactor decreases. Further, any scale that comes off the surface during the polymerization and eventually becomes mixed in the polymer product deteriorates the quality of the polymer. Furthermore, the removal of scale from the surfaces not only requires extensive labor and time but also gives rise to a serious health problem of workers as well as working environmental problems due to the presence of toxic, unreacted monomers absorbed in the scale.
The problem of the polymer scale deposition in the emulsion polymerization is especially formidable when the wall of the reactor and the stirrer are made of a metal, such as stainless steel, the very surfaces of which are exposed direct to the monomer or monomers. This is because polymer scale deposition on such surfaces is extremely large in amounts in the emulsion polymerization involving a dienic monomer, and may render it impossible to carry out the polymerization.
Methods for the prevention of polymer scale deposition have been proposed with respect to the polymerization of vinyl monomers, such as vinyl chloride, in an aqueous medium. According to the proposed method, the reactor walls and other surfaces are coated with certain polar organic compounds, amines, quinones and aldehydes as well as dyes, and the like (see, for example, U.S. Pat. No. 3,669,946). The method using those compounds for coating the surfaces is considerably effective in the polymerization of vinyl monomers, but it cannot be effectively applied to the emulsion polymerization of a dienic monomer or a mixture containing substantial amounts of a dienic monomer. Further, many of the compounds are colored compounds which tend to cause undesirable discoloration or staining to the polymer product. Furthermore, some of them are toxic and bring about a health problem to those handling the products.